Field of the Invention
The present invention relates to a trigger mechanism for a ball activated device for use in a well in the oil and gas industry. More particularly, the invention concerns a protective sleeve for use in such a trigger mechanism.
Background Art
In order to produce hydrocarbons, i.e. oil and gas, a borehole is drilled through several layers of rock in a formation. Hydro carbons may be present in a zone comprising a layer of porous rock under a layer of non-porous rock. Several such zones can be present along the borehole. The borehole may extend horizontally along one or more zones. All or part of the borehole can be lined by a steel casing or liner cemented to the rock to form a wellbore. One or more production strings can be inserted into the wellbore. As used herein, the term ‘tubing’ means any casing, liner or production string having a central bore through which a fluid may flow. Different tubings are provided with various devices such as valves, loggers, plugs, packers etc. in order to complete the well or to control the production from the different zones as known in the art.
One or more injection wells can be provided in a similar manner. An injection well is typically used to increase the pressure in a remote part of a zone to force the hydro carbons in the direction of a production well and thereby increasing the production.
The devices in the well can be operated in a number of known manners, including by so-called drop balls. A ball activated device is included in a tubing, and comprises a ball seat which forms a fluid tight obstruction with a drop ball of a suitable size. When it is desired to activate the device, the drop ball is dropped or pumped down within the tubing until it lands on the ball seat. Then, pressure is applied behind or upstream from the ball. When the force exerted by the pressure on the piston area exceeds a predetermined level, the ball seat shifts downstream and activates the device, for example by shifting a sliding sleeve valve from a closed position to an open position. In a cementing operation cement can then be pumped through the open valve into an annulus behind the casing, e.g. between the casing and the formation. In a fracturing operation, fracturing fluid with suitable proppants can be pumped through the open valve.
As known in the art, any suitable object can be dropped or pumped down the well to prevent fluid flow through a seat. The terms ‘ball’ and ‘ball activated’ are used for simplicity, and the term ‘ball’ should be regarded as any object capable of blocking a flow as discussed above.
In some wells, several ball activated devices are provided with seat diameters that decrease with the distance from the surface, which is termed the downstream direction in the present disclosure. To activate the ‘deepest’ device, i.e. the device furthest away from the surface, the smallest of a plurality of balls is pumped down and passes all the larger seat diameters before lodging or landing on the last seat. Thereafter, successively larger balls are used to activate the devices closer to the surface.
For simplicity, a sliding sleeve valve is used to illustrate a ball activated device in the following description. However, it should be understood that the ball activated devices considered in the present invention are not limited to sliding sleeve valves. For example, a linear motion is easily transformed to a rotation using helical shoulders between two sleeves or a rack and gear arrangement. Thus, an axially moving seat may turn an element around its axis, e.g. a ball in a ball valve or a plate in a butterfly valve.
U.S. Pat. No. 4,360,063 A (Kilgore) discloses a slide valve with a ball seat comprising lugs on collet fingers defing a ball seat. When it is desired to close the valve, a ball is dropped into a tubing and pressure is exerted to move the ball downward and close the slide valve. When the valve closes, the lugs expand into a groove and permit the ball to fall through the slide valve member. The lugs hold the slide valve in closed position. The spaces between the lugs on the collet fingers may be dimensioned to be of close tolerance or provided with resilient material to restrict or prevent flow therethrough and/or the ball may be made of resilient material or have a hard core with a resilient cover to inhibit or prevent flow of fluid through the collet fingers when the ball is seated on the fingers. In this manner, one ball can lodge on several seats, all having the same diameter, and activate corresponding valves one by one.
In U.S. Pat. No. 4,360,063 the seat is affixed to the sliding sleeve. Thus, the force exerted on the ball and seat must be sufficient to overcome an initial retaining force keeping the sliding sleeve open plus a friction force between the entire sliding sleeve and the surface within which it slides all at once. This friction force can be significant, in particular if the slide valve has been exposed to aggressive and/or contaminated well fluids for an extended period of time. Further, before the ball lands on the seat, particles in the well fluids or scaling may deposit in the groove into which the lugs are supposed to expand. If the lugs do not expand radially, the ball is prevented from passing through and the intended operation fails.
U.S. Pat. No. 8,215,401 B2 (Brække et al.) discloses a collet configured to slide axially within an inner sleeve, which in turn is configured to slide axially within an outer sleeve. The collet comprises longitudinal fingers. Initially the fingers form a ball seat and the collet is retained by a first release mechanism designed to release the collet from the inner sleeve when a first pressure exceeds a predetermined level. A second release mechanism is designed to release the fingers when the device is activated, e.g. when the valve has shifted from an initially closed to a final open state. Once released, the fingers flare out in order to permit the ball to pass.
One problem with the expandable seat of U.S. Pat. No. 8,215,401 B2 is the need for a second pressure greater than a first pressure in order to release the second release mechanism after the first release mechanisms to ensure proper operation of the device. In some applications, it might be advantageous to activate a device once a predetermined pressure is reached, and still be guaranteed that certain steps between the initial and final states are performed in a predetermined sequence to ensure proper transition from the initial to the final state.
Further, the collet fingers in U.S. Pat. No. 8,215,401 B2 are preferably spaced apart such that one collet can be configured to a desired ball seat diameter by mounting suitable lugs between the distal ends of the fingers and the surface in which the collet slides. However, in applications where a fluid containing particles, e.g. in cementing or fracturing operations, particles such as sand or proppant may enter between the fingers and settle behind them such that they do not flare out to let the ball pass.
In one embodiment disclosed in U.S. Pat. No. 8,215,401 B2, the first release mechanism comprises a head intended to slide over a small stopping shoulder. This head may require a space between two sleeves into which sand or proppant may enter. In general, particles may enter spaces between or behind sleeves and prevent proper operation of the expandable ball seat.
In other applications, an expandable ball seat is designed to stay in a production string for an extended period of time before being activated. In such applications, scaling and/or corrosion may cause similar problems. For example, scaling may build up between the sleeves or in exposed grooves and prevent the sleeve from moving axially or the ball seat from expanding radially. Corrosion may affect mechanical parts such as exposed shear pins or helical shoulders required for transforming a linear motion into a rotation. Hence scaling and corrosion might prevent proper operation of the trigger mechanism and/or the ball operated device triggered by the mechanism.
An object of the present invention is to solve at least one of the problems above.